Piano construction for sound amplification



L LUBEROFF Nov. 14, 1950 Filed Aug. 9, 1945 nunk Nov. 14,` 1950 LuBERoFFPIANO CONSTRUCTION FOR SOUND AMPLIFICATION 6 Sheets-Shet 2 Filed Aug. 9,1945 Nov. 14, 1950 L. LUBEROFF 2,530,251

PIANO CONSTRUCTIN FOR SOUND AMPLIFICATION Filed Aug. 9, 1945 l 6Sheets-Sheet 3 i F/Q.

Nov. 14, 1950 |m LUBEROFF PIANO CONSTRUCTION FOR SOUND AMPLIFICATIONFiled Aug. 9, 1945 6 Sheets-Sheet 4 Nov. 14, 1950 l.. LUBEROFF 2,530,251

PIANO CONSTRUCTION FOR SOUND AMPLIFICATION Filed Aug. 9, 1945 6Shets-Sheet 5 N O' @Miam/5f I z5 HW??? Lg www Nov. 14, 1950 L. LUBEROFF2,530,251

PIANO CONSTRUCTION FOR SOUND AMPLIFICATION 6 Sheets-Sheet 6 Filed Aug.9, 1945 ,57gl 7.. g 65 a 5 Patented Nov. 14, l9'50 UNITED STATES PATENTOFFICE PIANO CONSTRUCTION FOR SOUND AMPLIFICATION 3 Claims.

This invention relates to musical instruments, and more particularly toanovel piano-type instrument having the various characteristics andfeatures hereinafter described.

One object of the invention is to provide a novel piano which has theappearance and tone quality of a grand piano, but which differs from theusual grand piano in that it has a vertical sounding board of relativelysmall dimensions.

Another object of the invention is to provide a high-quality piano-typeinstrument which may be manufactured at substantially lower cost and maybe sold at a correspondingly lower price than a conventional grand pianohaving comparable tone quality.

A further object of the invention is to provide a piano having avertical chamber in which is disposed a vertical sounding board, andalso having a horizontal chamber and at least one air passage betweenthe two chambers, whereby sounds generated by the sounding board areconveyed to the horizontal chamber for amplification by the latter.

Still another object of the invention is to provide such a piano whichfurther includes means for electrically reproducing sounds from thevibrations of the sounding board, such means including a loud speakerwithin the horizontal chamber.

' A further object of the invention is to provide a multi-sectionsounding boardin 'such a piano. Other objects of the invention willbecome apparent as the description proceeds.

In the accompanying drawings, Fig. 1 is a pictorial perspective View ofthe \musical instrument provided by thepresent in,- vention, with thelid or cover raised; Fig. 2 is a planfview of the instrument wit the lidor cover removed to show the various mounting arrangement-employedaccording to l the invention; Y

Fig. 7 is an enlarged perspective view illustrate' ing one of theknee-operated volume control de`r vices employed according to theinvention;

- Fig. 8 is an enlarged fragmentary"perspective` view showing thepreferred form of the volume control devices;

Fig. 9 is an enlarged fragmentary sectional view of one of theknee-operated volume control levers, showing the mounting thereof; and

Fig. 10 is a diagrammatic illustration of the electrical system employedin the instrument, illustrating certain novel features thereof.

Referring first to Figs. 1 to 4, the musical instrument provided by theinvention comprises a piano i and a tail section 2 removably attachedtogether as will be described in further detail hereinafter. The piano Iis a complete instrument in itself and comprises a keyboard 3, avertical sounding board 4 (see Fig. 4), and keyactuated strikingdevices, one of which is shown in Fig. 4 and is designated generally byreference character 5. The various components are mounted within or onthe piano casing which is designated generally by reference character 6,and which is provided with supporting legs 1. As may be seen in Figs. 1and 4 the casing has a vertical rear portion 8 to accommodate thesounding board and the associated striking mechanisms. The soundingboard is vertically disposed at the rear within the casing portion 8which has a removable rear panel 8a, as shown in Fig. 3. The soundingboard 4 is mounted in the customary manner, extending downwardly fromthe pin block 9, and has the usual supporting ribs I0 on its rear faceor side, and the usual bridge I I on its front face. The strings I2extend between the upper and lower pins I3 and I4 over the bridge I I.The piano section is also provided with the usual foot pedals I5, whichperform their customary functions through suitable associated mechanism.

While the piano is of the vertical type, i. e. it has a verticalsounding board, its height is substantially the same as that,` of theusual grand piano. As clearly shown in Figs. l and 2, the tail section2, which is removably attached to the piano section, has substantiallythe shape and dimensions of the tail section of the usual baby grandpiano. Therefore, the instrument as a whole has the general appearancekof a grand piano.

Thertail section 2 is of shallow depth and large horizontal area, and'itextends rearwardly from e the kupper partfof the piano section, as showninv Fig. 3'. The tail section comprises a Vertical for' ward wall"l6,'acurved wall Il joined to the forward'wall, a floor 18, Yand ahinged lidI9. I' 'The forward wall IG- is removablysecured tol the'pin blockl -9"by 'means of'y suitable screws 2o, yand "isf also secured to the casingor housing structure of the piano section by additional screws 2l. Thewall I6 has openings 22 therein through which sounds are conveyed fromthe sounding board 4 into the chamber formed by the hollow tail section.The latter serves as a sound-amplifying chamber as will be furtherdescribed later. Thus, the portion 3 of the piano casing or housingconstitutes a sound-generating chamber by reason of the presence of thesounding board therein, while the hollow tail section constitutes asound-amplifying chamber which receives the generated sounds through theopenings 22. The hollow tail section is supported at its rear by meansof a supporting leg 23, which is removably secured to the tail section.

It will be noted that by virtue of the detachability of the pianosection, the hollow tail section, and the rear supporting leg, theinstrument may be readily moved or transported in secl tions.

As shown in Figs. 2 and 3, a rail 24 extends adjacent to the curved wallll. The purpose of this rail is to strengthen the entire tail sectionand also to support certain components of the instrument as describedhereinafter. The rail 24 is preferably formed of laminated material soas to increase its rigidity.

In order to produce sounds from the piano electronically as well asmechano-acoustically, a plurality of electrical pick-up devices 25 and25a (see Fig. 3) are mounted on the rear face of the sounding board li.These pick-up devices are mounted in a special manner, as will bedescribed later. In the specific illustration, two pick-up devices areemployed and are located so as to respond to vibrations in the low andhigh frequency portions of the operating frequency range. The pick-updevices (see Fig. 6) preferably take the form of conventionalelectro-magnetic pick-ups comprising an electromagnet and a movablemagnetic armature, but any other suitable type may be employed. Asuitable pick-up is that manufactured and sold by Amperite Co. under thetrade name of Kontak Mike. The pick-ups serve to generate electricalcurrents which are supplied to electronic amplifying devices mountedwithin the hollow tail section. As described hereinafter, the pick-upsare preferably mounted on dilferent sections of the sounding board forresponse to different frequencies.

Referring to the components or devices disposed within the hollow tailsection, an audio frequency amplifier 25 (see Fig. 2) is mounted onsuitable supports 2l and 38 carried by the supporting rail 2li and anauxiliary rail 29. A radio receiver is also mounted on the saidsupports. A phonograph 3l and recorder 32 are mounted on a platform 33(see Fig. 3). A loud speaker 34 is mounted on the iioor !8 which servesas a baille and which is free of the other devices mounted Within thehollow casing. A grill 35 serves to cover the loud speaker and theamplifier 26 as may be seen in Fig. l, and a second grill 36 serves tocover the radio receiver The electrical connections of the variousdevices will be described later.

The vertical panel 31 (see Figs. 1 and 4) at the front of the pianosection l and immediately adjacent the keyboard 3 is made to serve as acontrol panel on which various control elements are mounted. At theextreme left-hand end of this panel (Fig. 1) there is provided arotatable knob 38 which is connected by means of a flexible cable 39(Fig. 2) to the tuning condenser gang 40 of the radio receiver 30.Adjacent to the `knob 38 of the person playing the piano.

there are provided a plurality of push buttons 4l and an associatedtuner 42 which is electrically connected to the radio receiver andprovides for remote push button tuning. Adjacent the push buttons thereare a plurality of knobs designated generally by reference character 43,and at the right-hand side of the control panel there are other controlknobs designated generally by reference character 44. These variousknobs are employed to effect switching and volume control operations,which will be described hereinafter. Finally there is provided a controlknob 45 which enables micro-tuning of the phonograph speed controlmechanism 46 (Fig. 2) by means of a shaft 41 and suitable gearing 48.The shaft 4l extends through an opening in the pin block 9, as may beseen in Fig. 3. The specific purpose of this phonograph tuning controlwill be described later. Thus it will be seen that all the Various handcontrols are within easy reach of a person seated in the piano playingposition.

In addition to the above-mentioned hand controls there are also providedtwo levers 49 and 50 (see Fig. l) which are operable by the knees Theseknee control levers are adapted to operate volume control devices so asto control the volume of sounds reproduced by the loud speaker 34 duringthe playing of the piano either alone or in conjunction with the radioor phonograph. Figs. '7 to 9 illustrate the left knee control lever andthe associated volume control device, it being understood that the kneecontrol mechanisms are similar, being arranged for actuation in oppositedirections. As shown in Fig. 9 each knee control lever is pivotallymounted at the underside of the key bed 5| (see Fig. 4), which alsoserves to support the piano keys. A suitable mounting arrangement isshown in Fig. 9, wherein an inner cylindrical member 52 is iixedlysecured to the panel 5l, and an outer cylindrical member 53 is securedto the lever 49. The lower end of the cylindrical member 53 is closed bya disc 54 which is interlocked with the cylindrical member 53. A bolt 55is supported within the hollow cylindrical structure, and a helicalspring 56 encircles the bolt and has its ends anchored at 57 and 58 tothe relatively movable parts of the hollow cylindrical structure. Thelever 49 carries a gear sector 59 which is arcuately slotted at 6D toaccommodate a stop pin 5| mounted on panel 5l. Thus the stop pin and theassociated slot SU serve to limit the range of rotary movement of thegear sector about the pivot axis of the lever 49. The spring `56 servesto maintain the lever normally in the position for minimum volume.

The teeth 62 on gear sector 59 mesh with a pinion t3 mounted on therotatable shaft of the associated volume control device 64. The latteris also secured to the panel 5|, as shown in Fig. 4 by means of screws65. Preferably the volume adjustment is effected in steps or increments,and to this end the volume control device has a plurality of stationarycontacts 66 (see Fig 8) which are engaged by a wiping contact arm 6Tcarried by the rotatable shaft 68.

Mention has been made previously that at least two pick-up devices areemployed, one for the lowY frequency or bass notes, and the other forthe high frequency or treble notes. Fig. 5 shows the rear face of thesounding board with the pick-up devices attached thereto. In accordancewith a further feature of the invention, a division of the bass andtreble sections 'I0 and Il s effected by splitting the sounding board 4and/,or the bridge I I. Preferably both the sounding board and thebridge are split, as shown at 69. In the illustration, the split 69 doesnot extend the full height of the board, but it may do so if desired.The purpose of this feature is to prevent the bass and treble overtonesfrom running together objection-ably when the loud sustaining pedal isused and the piano is being played electronically. This objection isvery pronounced in the ordinary electronic piano. By dividing thesounding board and/or the bridgeinto sections, I have found that therunning together of the treble and bass overtones can be controlled orsubstantially eliminated. Contrary to the prevailing notion thatsplitting the sounding board and/or the bridge of a piano willdeleteriously aect its tone quality, I have found that the splitting ofthe board enhances the tone quality of an electric piano employingpick-ups on the sounding board.

As previously mentioned, I prefer to employ a certain method ofattaching the pick-up devicesv to the sounding board. In any electronicpiano employing one or more pick-up devices attached to thesoundingboard, it is very important that each pick-up shall remain in a fixedposition and shall be maintained at all times in iirm engagement withthe board. This has been difficult of achievement owing to the fact thatthe sounding board tends to swell or move with changes in weather andclimatic conditions and with Variations in temperature. In accordancewith the present disclosure, this diiculty is overcome by attaching eachpick-up device to the sounding board by means of an adhesive material,consisting of rubber, a resin derived from gum or wood rosin, a fillersuch as zinc oxide, and a softener such as lanolin. As shown in Figs. 5and 6, each pick-up device is secured to the sounding board by placingthe adhesive material 12 about and over the pick-up device and over asubstantial area of the sounding board. I have found that this adhesivematerial becomes hardened and effectively becomes a part of the soundingboard, which is due to a chemical reaction between the shellac or othercoating on the sounding board and the chemicals with which the adhesivematerial is treated. This method of attaching the pick-up devices isvery effective in providing permanent xation of said devices despite anyswelling or moving of the board that may takel place.

Reference may now be had to Fig. `which is a diagrammatic illustrationof the electric system embodied in the instrument. In addition to theaforementioned devices, the system includes a microphone 13 and arecording meter 14, these devices being shown in Fig. I. The radioreceiver hereinbefore mentioned is of conventional form, and forconvenience its components are represented in Fig. 10 by the blocks ofrectangles 30a and 38h'. The block 38a represents the radio frequencyand detector stages, while the block 38D represents the audio amplifiersection of the receiver. The phonograph 3l, hereinbefore mentioned, isalso of conventional form and is represented by a block or rectangle.The same is true of the amplifier 26' and the recorder 32. The necessarypower for operating the various devices is supplied in accordancewith'conventional vpractice, and for convenience n the power supply isrepresented in Fig. 10 by means of the rectangle 15, The diagram of Fig.l0 is a simplified single line diagram of the various electricalconnections.

The various switches shown and the various p6- tentiometers arecontrolled by means of the knobs 43 and 44 hereinbefore mentioned.

The pick-ups 25 and 25a are connected to a double-throw switch 16, as isalso the microphone 13. The pick-ups are preferably connected to theswitch through individual potentiometers or voltage dividers 11 and 11a,by means of which the outputs of the pick-ups may be relatively varied.Thus the amplitude of the electrical signal derived from each section ofthe sounding board may be controlled as desired. By means of the switch16, either the pick-ups or the microphone may be connected to thepre-amplifier 26, which in turn may be connected to the audio amplifier30h through the ganged double-throw switches 18 and 19. The left-handknee-operated volume control device, hereinbefore described, is shown at64, and an associated hand-operated Volume control device is shown at80. By means of the ganged switches 18 and 19, either of these volumecontrol devices may be rendered operative, re-v gardless of the positionof the hand-operated volume control device 80.

The phonograph 3| and the portion 30a of the radio receiver areconnected to a doublethrow switch 8l, by means of which either of thesaid devices may be connected to the audio amplifier 38h through theganged switches 82` and 83. The latter serve to control the righthandknee-operated volume control device 84 and the associated hand-operatedvolume control device 85. That is to say, by means of the gangedswitches 82 and 83, either of the said volumeL control devices may berendered operative, regardless of the previous position of either volumecontrol device.

The output of the audio amplifier 30h may be supplied to either the loudspeaker 34 or the recorder 32 by means of ganged double-throw switches86 and 81 (or a double-pole doublethrow switch). The connections to therecorder may include a transformer 88. The recording meter 14 may beconnected across the secondary winding of the transformer, as shown. Inaccordance with a further feature of the invention, provision is madefor using the loud speaker 34 as an audible indicator during recording.To this end, there are provided resistors 89 and 90 connected to theswitches 86 and 81 in the manner illustrated. When the switches are inthe upper manner to reproduce sounds emanating from one or more of thevarious devices employed in the instrument. When the switches 86 and 81are in the lower positions, the recorder is connected to the audioamplifier 30D, and the loud speaker is also connected to the amplierthrough the resistors 89 and 90. The values of these resistors are suchthat they permit a small percentage, preferably about 5%, of the soundcurrents to pass into the loud speaker, which thus serves as an audibleindicating device and enables the operator to tell when the recordingshould be started and stopped. volume of sound emanating from the loudspeaker is insufficient to cause mechanical feedback to the recorder.volume control devices and the recording meter 14, enables the operatorto control the recording any sound recording operation, the amplitude ofAt the same time, the

This feature, together with the 7 thesoundsgnals must be accuratelycontrolled to prevent under-cutting or over-cutting of the sound groovesin the recording disc, and this necessitates proper variation of theamplitude throughout the recording operation. The necessary accuratecontrol is enabled by means of the arrangement provided by the presentinvention.

The power supply unit 'l5 is energized-by means of the main power switchel, and it supplies the necessary power and voltages to the variousdevices through the connections S2, 93, 94- and S5, Switches 9E and Siserve to control the operation of the radio receiver and the phonographrespectively.

Referring new to the operation of the musical instrument, it may beassumed rst that it is desired to play the piano by itself and not as anaccompanimentto any of the other devices. In such case, the switches 96and Gl will be opened. The piano may be operated with or without theoperation of its associated electronic system. I the latter case, themain power switch SI1 will be opened. The casing or enclosure formed bythe hollow tail section 2 serves as asound-receiving chamber and itfunctions in the manner of a megaphone to amplify the sounds emitted bythe sounding board. The casing or enclosure thus functions as anacoustic amplifier and it also functions as a box-type baffle for theloud speaker 34. The acoustic output of the loud speaker is verysubstantially increased by reason of its being disposed within the saidenclosure. Further, the shape of the enclosure, and the location of thespeaker adjacent the rear curved wall thereof, contribute greatly to theeffects produced.

Usually it will be desired to utilize the electronic system associatedwith the piano in order to obtain the improved tone quality resultingtherefrom. In such case, the main power switch 9i will be closed, andthe switch 75 will be closed in its upper position to connect thepick-ups and 25a to the preamplifier 26. The switches 8S and 8'! willalso be closed in their upper position to connect the loud speakerdirectly to the amplifier 36h. The switches 'i3 and i@ may be closed ineither position, depending upon whether or not it is desired to use theknee-operated volume control device 6G. With the electronic system inoperation, the piano operates both mechano-acoustically andelectronically. Although the piano itself it relatively small, the

instrument as a whole is comparable to a high quality concert grandpiano from the standpoint of both volume and tonal quality. rhis hasbeen demonstrated by an instrument constructed as illustrated anddescribed herein.

The selective knee-hand volume control arrangement is a highly desirablefeature. If it is desired to play the piano electronically withoutelectrical variation of the volume, the ganged switches 'I8 and le maybe adjusted to their right-hand position merely by turning one of thecontrol knobs on the control panel, and the volume control device B maybe set to a desired volume level merely by adjusting another of thecontrol knobs. If it should be desired to vary the volume electricallyduring the playing of the piano, the ganged switches 'i8 and 19 may bethrown to their left-hand position simply by turning the appropriatecontrol knob. Thereafter the volume may be controlled by actuating theleft knee lever so as to produce any desired volume variation. Moreover,the pianist may switch from hand volume control to knee volume to thedesired volume level.

control', or vice versa, during the playing of a'- selection, since thismerely requires the turning of a single control knob, which can be donevery quickly and without interrupting the playing ofl thev pianoselection.

Whenever desired, the piano may be played as an-accompaniment to eitherthe radio or the phonograph, simply by closing one or the other of theswitches 96 and 91 and by properly positioning the switch 8l. In suchcase, the volume of the radio or phonograph reproduction may be set at adesired level, by throwing the switches 82A and 83 to their right-handposition and by setting handV volume control device 85 according Ifdesired, however, the volume level of radio or phonograph reproductionmay be varied by means of the knee-operated volume control device 84,merely by throwing the switches 82 and 83 to their left-hand position.Thus it is possible for one. to play the piano as an accompaniment toeither the radio or phonograph, and at the same time vary the respectivevolume levels of the piano and the accompanying instrument by operatingthe right and left knee control levers. It is possible in this way toobtain desirable and artistic musical interpretation.

When it is desired to play the piano as an accompaniment to thephonograph, the latter may be tuned to they pitch of the piano by meansof the knob d5. Rotation of this knob varies the speed of the.phonograph turntable and thus varies the pitch of the phonographreproduction. Since the knob 45 is located within easy reach of theplayer, the phonograph may be easily adjusted to the same pitch as thepiano by aural comparison while adjusting the said control knob. Sincethere is substantial resistance to the turning of knob 45, due toinherent friction in the actuating mechanism between the knob and thespeed control mechanism 46, this tuning arrangement not only enablesline exact tuning of the phonograph to the pitch of the piano while inplaying position, but also serves to maintain the steady desired pitchor tone of the phonograph, preventing the latter from getting out oftune."

With respect to the piano and phonograph and the associated controls,the instrument has utility features which might not be apparent to thecasual observer. There are now available recordings of piano concertos,and other compositions involving a piano part, in which the piano partis omitted so that it may be supplied as an accompaniment by the user.Such recordings, known as Add-a-Part records, are sold with printedmusic` showing the parts recorded and the part omitted. The volumecontrol and phonograph tuning features of the present instrumentespecially adapt it for the use of such recordings in conjunction withthe piano, since the pianist has absolute control of the reproductionbothA as to volume and pitch and he also has control of the pianovolume.

Moreover, by virtue of the features above mentioned, the instrument maybe used to advantage in the teaching or learning of piano. The pianopupil or student may practice with special recordings of the typeabove-mentioned whichV may be adapted for various degrees of advancementof the. pupil. In such case, the recording will serve as a musicalmetronone.

Whenever desired, the recorder 32 (Fig. 10) may be connected forrecording by throwing the ganged switches 86 and 81 to their lowerposition. A recording may be made during the playing of any of theseveral instruments or devices individually or in combination, by properpositioning of the switches, as will be apparent from the foregoingdescription. In addition it is also possible to reproduce or recordvoice from the microphone 13 either alone or as an accompaniment to theradio or phonograph by proper positioning of the switches. During anyrecording operation, the knee-operated volume control devices may beutilized to advantage, as previous- 1y described.

It is important to note that recording of piano is effected directlyfrom the sounding board by means of the pick-ups and the amplifiersystem. This is much more satisfactory than the conventional method ofrecording piano by means of a microphone, which has never been veryysatisfactory, especially since piano recording is most difilcult. Thepresent invention provides a solution of this problem.

From the foregoing description, it will be seen that the inventionprovides a novel musical instrument which is fundamentally an electronicpiano, but which combines various features that greatly extend itsutility and make it an al1-pur pose instrument. It will be apparent, ofcourse, that the invention is not limited to the various structuraldetails but is susceptible to modification within the scope of theappended claims.

I claim:

1. A piano comprising a walled structure forming a vertical chamber,said structure having front and rear vertical walls of substantial widthand height, and relatively narrow side walls extending between the frontand rear walls, whereby the vertical chamber defined by said walls is ofsubstantial width and height but is of relatively narrow dimension fromfront to rear, a vertical sounding board and associated vertical stringsdisposed within said chamber, a hammer mechanism also disposed withinsaid chamber in cooperative relation with said strings, a key boardextending forwardly from the front of said chamber at the upper partthereof and including keys operatively connected with said hammermechanism, a second walled structure forming a horizontal chamberextending from the rear of said first chamber at the upper part thereof,Said horizontal chamber being f substantial horizontal area but ofrelatively small vertical dimension, supporting means for said secondstructure at the rear thereof, and at least one air passage between saidvertical chamber and said horizontal chamber, whereby sounds generatedby the sounding board in said vertical chamber are conveyed by saidpassage to said horizontal chamber for amplification by the latter.

2. A piano comprising a walled structure forming a vertical chamber,said structure having front and rear vertical Walls of substantial widthand height, and relatively narrow side walls extending between the frontand rear walls, whereby the vertical chamber defined by said walls is ofsubstantial width and height but is of relatively narrow dimension fromfront to rear, a vertical sounding board and associated vertical stringsdisposed within said chamber, a hammer mechanism also disposed withinsaid chamber in cooperative relation with said strings, a key boardextending forwardly from the front of said charnber at the upper partthereof and including keys operatively connected with said hammermechanism, a second walled structure forming a horizontal chamberextending from the rear of said first chamber at the upper part thereof,said horizontal chamber being of substantial horizontal area but ofrelatively small vertical dimension, supporting means for said secondstructure at the rear thereof, at least one air passage between saidvertical chamber and said horizontal chamber, whereby sounds generatedby the sounding board in said vertical chamber are conveyed by saidpassage to said horizontal chamber for amplification by the latter, andmeans for electrically reproducing sounds from the vibrations of thevertical sounding board, said last means including a loud speakermounted within the horizontal chamber on a part of said second walledstruc-` ture which acts as a baffle.

3. A piano according to claim 2, wherein the Vertical sounding boardcomprises a plurality of sections which are vibrational substantiallyindependently of each other, each section covering a predeterminedportion of the audio frequency range covered by the entire soundingboard, and wherein individual Vpick-ups are provided on the respectivesections of the sounding board.

LOUIS LUBEROFF.

REFERENCES CITED The following references are of record in the le ofthis patent:

UNITED STATES PATENTS Number Name Date 1,681,161 Baldwin Aug. 21, 19281,739,680 La Joie Dec. 17, 1929 1,915,177 Berghane June 20, 19332,001,722 Hammond, Jr May 21, 1935 2,078,385 Kato Apr. 27, 19372,200,718 Miessner May 14, 1940 2,219,539 Riechers Oct. 29, 1940 FOREIGNPATENTS Number Country Date 491,985 Great Britain of 1938 OTHERREFERENCES Pub., Radio-Craft for January 1939 (pages 398 and 433)

